In recent years, constant on-time (COT) structure is applied in wide-input-voltage-range systems generally. However, the stability of COT structure is always suffered from limitations on the effective series resistance (ESR) of output capacitors. As shown in FIG. 1, a COT structure 10 in common use includes a controller 12 to provide control signals UG and LG to switch the serially connected power switches SW1 and SW2 in a switching circuit 14, in order to modulate the output inductor current IL flowing through an output inductor L1 and charging an output capacitor COUT to generate an output voltage Vo. The control signals UG and LG have pulse widths determined by the input voltage Vin and the output voltage Vo while a constant frequency within the whole input voltage range. The controller 12 relies on the ESR of the output capacitor COUT to act as a current sense resistor, so the ripple of the output voltage Vo provides the pulse width modulation (PWM) ramp signal. The pulse width of this one shot is determined by the input voltage Vin and the output voltage Vo to keep the frequency fairly constant over the input voltage range. “Double-pulsing” is a way to identify the unstable operation of a COT structure and occurs due to noise on the output or because the ESR of the output capacitor COUT is too low that there is not enough voltage ramp in the output voltage signal. For this reason, a capacitor with very small ESR, such as a ceramic capacitor, is not applicable to the output capacitor of a COT structure.
Therefore, it is desired a solution that can make a COT structure stable by using ceramic output capacitors and no longer affects the equivalent value of combination of the ESR of the output capacitor and capacitive load.